The incapacity of a pilot when piloting an aircraft is manifested by an abnormal behaviour of the pilot for several seconds, in particular marked by a lack of vigilance of the pilot which may be due to physical or psychological illness (stress). As soon as a suspicion of incapacity when piloting exists, a plan of action is initiated to replace or assist the pilot so as to continue the flight of the aircraft under optimal conditions.
During a flight, the state of vigilance of a pilot is measured by another pilot, the ground control with which the pilot is in permanent radio contact, or else by sensors worn by the pilot (cardiac sensors for example). The wearing of such sensors for several hours of flight may inconvenience pilots.